1. Field of the Invention
The present invention relates to: a press-forming method of a tubular part having a cross section of an irregular shape used for underbody parts, such as an axle beam; and a tubular part having a cross section of an irregular shape formed by this press-forming method, for a vehicle, such as an automobile or track.
This application is a national stage application of International Application No. PCT/JP2009/058832, filed May 12, 2009, which claims priority to Japanese Patent Application No. 2008-124787, filed on May 12, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
Tubular parts having a cross section of an irregular shape are conventionally and widely used as underbody parts for vehicles, such as axle beams disposed between the wheels of the vehicle and their surrounding suspension parts.
Since the tubular parts having the cross section of the irregular shape repeatedly takes an impact load, a torsional load, and the like while the vehicle is moving, high fatigue resistance is required in addition to high strength. These days, it is required that underbody parts for vehicles, such as axle beams, are not formed using a plurality of parts, but formed by press-working one steel tube.
An axle beam of a hollow structure formed into a cross section of an irregular shape by press-working of a high-strength steel tube is disclosed in the Japanese Unexamined Patent Application, First Publication No. 2001-321846.
However, in the axle beam of the Japanese Unexamined Patent Application, First Publication No. 2001-321846, heat treatment, such as quenching, is performed in order to control the residual stress in a stress concentration zone and to increase the fatigue resistance of the material. Moreover, an atmosphere control or descaling process is required to prevent oxidized scales formed by the heat treatment. As a result, there are problems in that not only the cost of the heat treatment but also the extra cost of coping with the above oxidized scales is incurred.
In addition, there are also problems in that the heat treatment deteriorates product precision and makes the attachment operation to the vehicle body difficult.